An understanding of how and why proteins fold and of the nature of the interactions required to confer a unique stable folded structure remains a central objective of structural biochemistry The research described in this proposal is aimed at elucidating the minimum set of interactions required to stabilize folded and partially folded protein domains. Solid state NMR will be used to study the geometry in several labeled amino acid analogs that can serve as models for low barrier hydrogen bonds (LBHBs) between amino acids. High field solution NMR will be used to study the role a novel hydrogen-bonded network plays in stabilizing the structure of the peripheral subunit-binding domain, one of the smallest examples of a protein that is able to adopt a stable, compact fold. NMR will also be used to study the structure of a modified ribosomal protein, LP. These studies will help to elucidate the interactions required to stabilize folded and partially folded states of proteins.